There have been proposed many methods for forming a low-reflection film on a substrate in the field of cathode-ray tube for TV and computer terminals, not to mention sheet glass, optical parts and optical instruments.
Conventional methods consist of coating the surface of a cathode-ray tube with an SiO.sub.2 layer having minute irregularities, thereby producing the anti-glare effect, or consist of etching the surface of a cathode-ray tube with hydrofluoric acid, thereby forming surface irregularities, as disclosed in Japanese Unexamined Patent Publication No. 118931/1986. The coating and etching by these conventional methods are called non-glare treatment because they merely form minute irregularities which reflect and scatter the incident light; they are not essentially designed to form a low-reflection film and hence they have a limit in decreasing the reflectivity of the substrate.
On the other hand, attempts have been made to coat the surface of lens or glass with a thin film of MgF.sub.2 (which is a stable substance having a low refractive index) by vacuum deposition. Unfortunately, vacuum deposition has a disadvantage of requiring an expensive apparatus and involving difficulties in handling large objects (such as finished cathode ray tube and sheet glass) and a large number of objects for mass production. All this leads vacuum deposition to high production cost.
The present invention was completed to eliminate the above-mentioned disadvantages involved in the conventional technology. Accordingly, it is an object of the present invention to provide a new method for forming on a substrate a stable, transparent thin film having a low refractive index in a simple chemical manner. It is another object of the present invention to provide a new method for forming on a substrate a thin film having the low-reflection characteristics.